Chemical duty pumps require corrosion-resistant materials to be used. Non-chemical duty pumps, particularly high vacuum, oilless vacuum pumps, have been diaphragm pumps and one of the designs utilizes an aluminum pin press fit into an aluminum valve plate to hold a flapper valve which operated as the pump stroked. The diaphragm hit the pin on each stroke, but since the pin was held firmly in the valve plate by the tight press fit, that was harmless and acceptable.
In chemical duty pumps, the materials of the valve plate must be corrosion resistant and therefore the material of the valve plate is made of PTFE, commonly known as Teflon™. The pins also must be corrosion resistant, but since they are subjected to the stresses of holding the flapper valve, which is stretched over them, they are made of a harder plastic material, for example, glass- or graphite-filled polyetheretherketone (“PEEK”). The valve itself is typically a corrosion resistant elastomer, for example, Kalrez™. Typically the valve is a flat rubber valve attached to the valve plate with two pins. The valve has two oblong holes for the pins to allow movement needed to open and close the valve.
The diaphragm contacting the pins on each stroke causes the pins to become sunk into the valve plate, which adversely affects the operation of the valve. The valve opening may be restricted or it may not be able to completely close. The press fit in the PTFE is not sufficient to prevent this condition.